The loss of B-cell tolerance is central to the pathogenesis of autoimmunity in SLE. In order probe the mechanisms involved in this process, we have utilized the chronic graft versus host model of SLE in which we transfer MHC class II-incompatible bm12 spleen cells or purified CD4 splenic T cells to C57BL/6 mice, utilizing various recipient transgenic variants. This system allows us to investigate in detail the mechanism of loss of B-cell tolerance that characterizes SLE. Our previous work on this project and our recent preliminary data has identified several exciting directions that we are proposing to pursue in the current application. We will address questions such as: Why does SLE make B cells resistant to depletion with mAb? What role does do receptor editing and allelelic inclusion play in the loss of B cell tolerance? How do CD4 T cells control the ontogeny of B cells, so that they can respond to alloreactive T cells? What is the phenotype and distribution of autoantibody forming cells, and how does that relate to clinical disease? We will continue this work with four specific aims: (1) What are the mechanisms of B-cell depletion as a potential therapy for SLE? (2) At what point in ontogeny can B cells lose tolerance? (3) How do CD4 T-cells influence the ontogeny of B cells? (4) Where are autoantibody forming cells found in SLE, particularly as regards the kidneys? The better understanding of the role of B cells in systemic autoimmunity and the therapeutic effects of B-cell depletion will permit the more efficient use of existent B-cell targeted therapies, such as rituximab, and the more rationale development of newer B- cell therapies or combinations of therapies. It is likely that new biomarkers will need to be identified to allow the rational monitoring of B-cell directed therapies and the appropriate selection of patients who are likely to respond. This project will use an experimental mouse model of the complex autoimmune disease systemic lupus erythematosus. The studies will particularly investigate the role of one of the key cells involved in this disorder: the B lymphocyte. The knowledge gained will improve of our understanding of the pathogenesis of systemic lupus erythematosus and how we can target the B lymphocytes in treatment.